Electrically heating oil reservoirs is known and is usually practised to modify the mobility of the oil near the well-bore and to improve fluid transmissibility through the near-wellbore region. The reduced pressure in the near-wellbore region causes the oil in the region to lose light ends and develop increased viscosity. This region is referred to as the "visco-skin" and can significantly reduce production. By electrically heating the oil near the wellbore, the viscosity may be reduced and the visco-skin effect may be removed. Waxy hydrocarbons may also be sufficiently mobilized to aid in increased production.
In electrical heating of wells, it is conventional to:
drill a vertical well into the oil reservoir and case it to the interface of the overburden and oil reservoir; PA1 install an electrode assembly in the well to extend into the reservoir from the foot of the casing, the assembly comprising an upper non-conductive tube (termed an "isolator"), a conductive tube (the electrode), and a bottom isolator, the electrode being in contact with or electrically coupled to the reservoir; PA1 install a string of tubing in the casing, electrically isolated from the casing by annular dielectric centralizers, the tubing being electrically connected with the electrode by a conductive bow spring device; PA1 the tubing string being connected at ground surface to the positive lead of a power conditioning unit, so that AC current is supplied down the tubing and through the bow spring device and electrode into the reservoir; PA1 the casing being connected to the negative lead of the power conditioning unit, whereby the current flows from the electrode, up through the near-bore region of the reservoir to the casing and up the casing to ground. PA1 the relatively small sphere of heating; PA1 having physical limits to the maximum current levels; and PA1 creating high flow velocities, requiring large compensatory current levels to heat the reservoir. PA1 That when attempting to heat the reservoir adjacent a 500 meter long horizontal well (electrode), the great volume of reservoir affected diminishes the reservoir resistance to 1/4 to 1/8 of the combined resistive loads of the power delivery and ground return systems. Thus the reservoir resistance becomes an alteration of the smallest of the circuit resistances. Using the single wellbore technology of the prior art vertical well, the efficiency of converting electrical energy to heating the reservoir would fall from about 80% to 10 to 25%; and PA1 That the efficiency is so poor, that to heat the reservoir electrically would require extremely high currents that could not be practically or economically attainable within the limits of the current state of the art. PA1 a plurality of vertical wells, each having a wellbore extending into the reservoir and being cased down to the upper end of the reservoir; PA1 a power conditioning unit ("PCU") located at each vertical well; PA1 each vertical well having a supply electrode in electrical contact with the reservoir; PA1 conductive means, such as a tubing string, connecting the positive lead of the PCU with the supply electrode, for supplying alternating current to the reservoir through the electrode; PA1 a horizontal well having a wellbore consisting of a vertical riser leg and a horizontal liner leg, the liner leg extending through the reservoir in contiguous but spaced relation to the vertical wells, said riser leg being cased; PA1 said liner leg containing a conductive apertured conduit or liner in electrical contact with the reservoir, said liner forming a return electrode extending substantially the length of the liner leg; PA1 said riser leg containing conductive means (e.g. a tubing string) connected with the liner and the negative lead of the PCU; PA1 each electrode being electrically isolated by non-conductive means from its associated casing string. PA1 That heat transfer into the reservoir by thermal conduction was a desirable feature which is best accomplished with a low fluid inflow, characteristic of horizontal wells but which is a liability with respect to the capability to cool high current loads; PA1 That it was desirable to keep the supply electrode lengths as short as possible to keep the power conversion efficiency high. This was not feasible with a single wellbore, dual electrode, long horizontal well, and thus a plurality of vertical supply electrode wells are provided; PA1 That using the horizontal well as the return electrode converted the ground return system losses to useful reservoir resistance and increased efficiencies back up to 40 to 60%; PA1 That it was necessary to conduct high current into the large reservoir yet it was desirable to keep the current levels low per unit length of horizontal well, due to the low cooling capabilities of the characteristically low fluid flows. This was solved by providing multiple supply electrodes and staging the current flow in smaller discrete amounts into the horizontal well liner. As the accumulating current requires greater cooling, the accumulating volumetric flow correspondingly increases, adequately meeting the demand; and PA1 That as produced liquid rates dropped at the vertical wells, current would need to be reduced limiting the heating and production. However, as there is a horizontal producer, it is a possibility to extend production from the horizontal well by converting the vertical wells to water flood injectors to maintain adequate cooling for the required current while simultaneously flushing residual oils to the horizontal production well. PA1 supplying current to a plurality of electrodes, each being disposed in one of a plurality of vertical wells, each electrode being in electrical contact with the reservoir, so that the current enters the reservoir; and PA1 returning the current through the conductive liner and tubing string (or cable) of a horizontal well extending into the reservoir in spaced relation from the vertical wells. PA1 1. more efficient heating of the reservoir by minimizing losses in the liner, tubing and casing string; and PA1 2. more uniform heating of the reservoir adjacent to the horizontal well by minimizing any wavelength effects which are a strong function of the frequency.
Thus the electrical circuit used to do electrical heating consists of the power conditioning unit, the power delivery system (tubing and bow spring device), the electrode, the reservoir, and the return system (casing).
The withdrawal of fluids from the reservoir by way of the well usually occurs at the same time as electrical heating.
Generally, at practical current levels, the current density distribution may be sufficient to only heat the reservoir within about 5 to 10 meters radially from the electrode.
With most wells, the tubing string and casing are usually short and conductive enough that the largest part of the resistive load is in the reservoir. The reservoir resistance is typically 5 to 10 times larger than the combined resistances of the power delivery and ground return systems. This means that the majority of the electrical current is dissipated as heat in the reservoir and good power conversion efficiencies are achieved.
Despite the relatively high conversion efficiency of the prior art system, several disadvantages and limitations are related to the high amperages used.
First, delivery of the high current to the electrode is a significant consideration. If one uses cable instead of the tubing as part of the power delivery system, the cable is significantly de-rated due to its submerged condition and is limited to a current of less than 100 amperes before the cable may be damaged. Current levels of less than 100 amperes severely restrain the commercial application of the electrical heating process. A preferred approach is to use the tubing string itself which, even though it is a poorer conductor, is significantly cooled by the produced liquids from the reservoir. Use of the tubing string in an environment with cooling provided from the produced fluids, increases the current constraint of the power delivery system to more than 1000 amperes. The maximum current is therefore dependent upon the rate of fluid flow in the tubing.
Additionally, increased amperages of alternating current result in correspondingly higher hysteresis losses in magnetic conductors, such as the tubing string. The hysteresis losses manifest as energy losses that are not then available to heat the reservoir. Hysteresis losses may be controlled by reducing the frequency of the applied source of alternating current.
Further, the relatively high removal rate of heated oil, characteristic of vertical well production rates, places large heat loss demands on the formation, requiring relatively high sustained heating and thus high current levels.
In summary the disadvantages of the electrically heated vertical well system include:
There have been attempts by others to utilize horizontal well techniques (to involve greater portions of the reservoir), in combination with electrical heating techniques of the single wellbore approach described above. These efforts have suffered significant reductions in heating efficiency and ultimately supply only low levels of heating to the reservoir. Particularly, alteration of the single vertical well technology to horizontal well technology suffers the following disadvantages:
With this background in mind it was the objective of the present invention to provide an electrically stimulated well arrangement and technique that would have increased influence on the reservoir, more effective use of the current supplied and result in improved production rates.